The invention generally relates to laser drilling.
It is known from EP-A-0 164 564 to produce blind bores in a laminate comprising the layer sequence metal-dielectric-metal with the aid of an excimer laser. The top metal layer of the laminate is in this case used as a perforated mask, the pattern of perforations in which is transferred photographically and is produced by subsequent etching. The dielectric which is uncovered in the region of the hole of this mask is then removed by the action of the excimer laser until the bottom metal layer is reached and the process of removing material is terminated. The known method is used in particular to produce the required through-contact holes in the form of blind bores during the fabrication of multilayer circuit boards.
German journal xe2x80x9cFeinwerktechnik and Messtechnik 91 (1983) 2, pp. 56-58 has disclosed a similar method for the fabrication of multilayer circuit boards in which the blind bores which are used as through-contacts are produced with the aid of a CO2 laser. In this case too, the top copper foil is used as a perforated mask, in which the copper is etched away wherever the laser beam is to produce a hole.
DE-A-197 19 700 has also already disclosed devices for the laser drilling of laminates in which a first laser, having a wavelength in the range from approximately 266 nm to 1064 nm, is used to drill the metal layers and a second laser, having a wavelength in the range from approximately 1064 nm to 10600 nm, is used to drill the dielectric layers.
U.S. Pat. No. 5,593,606 has disclosed a method for the laser drilling of laminates in which a single UV laser, the wavelengths of which are below 400 nm and the pulse widths of which are below 100 ns, is used to drill the metal layers and to drill the dielectric layers. Therefore, on the condition that an excimer laser is not used, metal and organic material are drilled using the same UV laser.
DE-A-198 24 225 has disclosed a further method for the laser drilling of laminates, in which, by way of example, an SHG (second harmonic generation) YAG laser with a wavelength of 532 nm or a THG (third harmonic generation) YAG laser with a wavelength of 355 nm can be used to drill the metal layers and to drill the dielectric layers.
It can fundamentally be stated that, when UV lasers, i.e. lasers with wavelengths of below 400 nm, are used for the laser drilling of organic materials, photochemical decomposition of the organic materials takes place. Therefore, there is no burning and, on account of the at most extremely low thermal load, there is no delamination when used for laminates. By contrast, when CO2 lasers are used for the laser drilling of organic materials, thermal decomposition of the organic materials does occur, i.e. burning may occur and there is a risk of delamination in the case of laminates. However, compared to UV lasers, CO2 lasers can achieve considerably shorter processing times when drilling organic materials.
An embodiment of the invention is based on the problem of allowing rapid production of blind bores or through-holes during the laser drilling of laminates which have at least one metal layer and at least one dielectric layer comprising an organic material without the laminates being damaged.
An embodiment of the invention is based on the discovery that frequency-doubled Nd vanadate lasers with a wavelength of 532 nm at short pulse widths of less than 40 ns can be used to effectively process both the metal layers and the dielectric layers.
In one case, pulse frequencies of xe2x89xa730 kHz can be selected for the laser drilling of the metal layers, while pulse frequencies of xe2x89xa720 kHz can be selected for the laser drilling of the dielectric layers. The selection of relatively high pulse frequencies for the processing of the organic materials results in particularly effective laser processing of both types of material. The laser processing of the organic materials results in a combination of photochemical and thermal decomposition which, compared to UV lasers, allows shorter processing times to be achieved and, compared to CO2 lasers, avoids excessively high thermal loads.
The frequency-doubled Nd vanadate laser which is selected according to an embodiment of the invention for the drilling of laminates allows very high pulse frequencies, which may even be over 100 kHz, at low pulse widths of less than 40 ns. The high pulse frequencies allow rapid and effective processing of the laminates, while the low pulse widths ensure that the thermal load is very low. This combinastion of high pulse frequencies and short pulse widths cannot be achieved with other lasers which operate with similar or identical wavelengths. For example, in the case of the SHG YAG laser which is known from DE-A-198 24 225, at relatively high pulse frequencies it is at best possible to achieve pulse widths of 70 to 80 ns.
One configuration , through the use of pulse widths of less than 30 ns, allows the thermal load on the laminates during laser drilling to be reduced still further.
When using a focused laser beam with a spot diameter of between 10 xcexcm and 100 xcexcm, the laser processing of metal and organic material is effective. When using spot diameters of between 20 xcexcm and 50 xcexcm the laser processing of the two materials can be made even more effective.
On account of the higher absorption of the laser beams in the organic material, one configuration allows the processing rate to be increased considerably. The additives should have a significantly higher absorptance for laser beams with a wavelength of 532 nm than the pure organic material.
One refinement allows a particularly simple and economic increase in the absorptance of the organic material to be achieved.
One configuration, on account of the choice of red additives, allows the absorptance to be optimized, since the green light of wavelength 532 nm is absorbed particularly well by the complementary color red.
One refinement provides, for the admixing of pigments as additive, a quantitative range which has proven particularly successful at increasing the absorptance without impairing the other properties. A narrower quantitative range can be regarded as optimal.
If the absorptance of the organic material is increased to at least 50% by the admixing of additives, the processing rate in the organic material has already been increased considerably. If the absorptance is increased to at least 60%, or to at least 80%, the processing times for the laser drilling of the organic material can be reduced further to a corresponding degree.